Support for move semantics is implemented using Boost.Move. If rvalue references
are available it will use them, but if not it uses a close, but imperfect
emulation. On such compilers you'll need to use Boost.Move to take advantage
of using movable container elements, also note that:

Non-copyable objects can be stored in the containers, but without support
for rvalue references the container will not be movable.

C++11 introduced a new allocator system. It's backwards compatible due to
the lax requirements for allocators in the old standard, but might need some
changes for allocators which worked with the old versions of the unordered
containers. It uses a traits class, allocator_traits
to handle the allocator adding extra functionality, and making some methods
and types optional. During development a stable release of allocator_traits wasn't available so an
internal partial implementation is always used in this version. Hopefully
a future version will use the standard implementation where available.

The member functions construct,
destroy and max_size are now optional, if they're not
available a fallback is used. A full implementation of allocator_traits
requires sophisticated member function detection so that the fallback is
used whenever the member function call is not well formed. This requires
support for SFINAE expressions, which are available on GCC from version 4.4
and Clang.

On other compilers, there's just a test to see if the allocator has a member,
but no check that it can be called. So rather than using a fallback there
will just be a compile error.

propagate_on_container_copy_assignment,
propagate_on_container_move_assignment,
propagate_on_container_swap
and select_on_container_copy_construction
are also supported. Due to imperfect move emulation, some assignments might
check propagate_on_container_copy_assignment
on some compilers and propagate_on_container_move_assignment
on others.

The use of the allocator's construct and destruct methods might be a bit
surprising. Nodes are constructed and destructed using the allocator, but
the elements are stored in aligned space within the node and constructed
and destructed by calling the constructor and destructor directly.

In C++11 the allocator's construct function has the signature:

template<classU,class...Args>voidconstruct(U*p,Args&&...args);

which supports calling construct
for the contained object, but most existing allocators don't support this.
If member function detection was good enough then with old allocators it
would fall back to calling the element's constructor directly but in general,
detection isn't good enough to do this which is why Boost.Unordered just
calls the constructor directly every time. In most cases this will work okay.

pointer_traits aren't used.
Instead, pointer types are obtained from rebound allocators, this can cause
problems if the allocator can't be used with incomplete types. If const_pointer is not defined in the allocator,
boost::pointer_to_other<pointer,constvalue_type>::type
is used to obtain a const pointer.

Since the containers use std::pair
they're limited to the version from the current standard library. But since
C++11 std::pair's piecewise_construct
based constructor is very useful, emplace
emulates it with a piecewise_construct
in the boost::unordered namespace. So for example, the
following will work: